Enable custom event masks for native windows

Enable custom event masks for native windows

Some apps really need to set custom event masks on native child windows,
for example emacs sets the event masks with gdk, but then reads out
the raw X events via a filter, so gdk event emulation doesn't work for that.

When we get motion or button events we map back from the event position and
window to the toplevel before doing anything, because a toplevel native window
could e.g. overlap a child window or whatever.